The invention relates to a method for beam guiding of a light beam generated by a light source, in particular laser light source, to a target location along a path distance LS.
The invention furthermore relates to an optical arrangement for beam guiding of a light beam generated by a light source, in particular laser light source, to a target location along a path distance LS.
A specific application described in connection with the present invention is the use of the method mentioned in the introduction and of the optical arrangement mentioned in the introduction in semiconductor lithography.
In semiconductor lithography, laser light is used to image a mask onto a wafer by means of a projection exposure installation in order then to fabricate the desired semiconductor components lithographically from the wafer. In this case, particular attention is given to the beam guiding system that guides the light beam generated by the laser to the projection exposure installation.
Owing to the demand for very short-wave light, semiconductor lithography at the present time uses excimer lasers, by way of example, which generate a light beam constructed, by its nature, from laser pulses of very short temporal delimitation with the individual laser pulses having a length of a few 10 ns. Each pulse of the light beam has a high pulse energy of, by way of example, more than 5 mJ. This means that, as seen over the duration of an individual pulse each pulse has a very high power density. These high power densities can damage the downstream optical systems or shorten the service life thereof.
Therefore, it is necessary to increase the pulse duration of the laser pulses or to split each individual pulse into a plurality of temporally offset subpulses with no loss of energy, in order thus to lower the power density.
Optical delay lines are usually used for pulse multiplication or pulse elongation. For this purpose, the light beam generated by the light source, that is to say by the laser in the present application example, is split into two partial beams. In addition to the path distance LS, one of the two partial beams correspondingly passes through the delay line with the path distance LD, which is also to be understood to mean that one partial beam passes through the delay line a number of times, so that the total delay path distance LD is then attained. At the end of the delay line, the two partial beams are then recombined to form a single light beam that is guided further to the target location, the exposure installation in the present example.
In this case, the problem exists that the light beam generated by the light source naturally has a beam divergence, i.e. becomes wider and wider as the path distance increases. On the other hand, the beam reaching the target location ought not to exceed a maximum beam width B in order to ensure a complete utilization of the light beam at the target location, i.e. in the exposure installation in the present case.
In order to achieve a sufficient reduction of the peak power of the individual pulses, however, large path distances LD are required for one partial beam. Large delay path distances LD have the effect, however, that, on account of the natural divergence of the light beam, the delayed partial beam at the output of the delay line has a larger beam width than the non-delayed partial beam.
Therefore, previous optical arrangements for beam guiding, i.e. beam guiding systems, use delay lines in which the input of the delay line is imaged 1:1 onto the output of the delay line, and which thus ensure that the beam properties of the delayed beam essentially correspond to those of the undelayed beam. Upstream of the input of the delay line or downstream of the output of the delay line, the light beam generated by the laser is additionally shaped to the desired beam width B, said light beam usually being expanded since the light beam generated by the laser generally has a beam width that is less than the desired beam width B at the target location, i.e. at the exposure installation.
The use of the abovementioned delay lines having an imaging optic that images the input of the delay line onto the output of the delay line has the disadvantage, however, that additional focusing elements are required in the delay line, which constitute an additional cost expenditure. Furthermore, these imaging optics within the delay line generate intermediate foci which constitute a high radiation burden per volume and accordingly have to be well purged or arranged remote from optical elements. Moreover, the optical elements, for example folding mirrors, present in the delay line are burdened to a greater extent by the focusing.
Beam guiding systems of this type are disclosed in the documents U.S. Pat. Nos. 6,549,267 B1, 6,535,531 B1 and 5,661,748 by way of example, which differ through the type of imaging optics in the delay lines. A refractive imaging optic, by way of example a Keppler telescope, is used as the imaging optic in the case of the document cited first, while the imaging of the input onto the output is achieved by means of spherical mirrors in the case of the documents cited last.
If, by contrast, imaging within the delay line is dispensed with, the disadvantage already mentioned arises in that the delayed partial beam, due to the natural divergence of the laser radiation and due to the longer propagation path via the delay line, no longer falls within the specified beam width range, but rather is increasingly fanned out over the propagation path, with the result that the predetermined beam width B desired at the target location is exceeded. Moreover, the recombined light beam thus has a greatly varying width over the entire lengthened pulse duration because the delayed partial beam has a greater beam width than the non-delayed partial beam. This has the effect that, at the target location, the light beam is not homogeneous over the cross section, as desired, but rather is inhomogeneous.